In the field of vehicle suspensions, the phrase "quarter car suspension" refers to the components of the vehicle suspension relating to one of the four wheels of the typical automotive vehicle. These components include the particular wheel with a tire that is in contact with the road, a spring that transfers the road force to the vehicle body (sprung mass) and suspends the vehicle body, and a damper or actuator that reduces undesirable relative movement between the vehicle body and wheel. The complete suspension system of an automotive vehicle comprises four quarter car suspensions.
In recent years, vehicle manufacturers have dedicated significant effort to developing suspension systems responsive to the driving conditions of the vehicle. This effort is triggered by a desire to incorporate the best features of soft and stiff suspension systems into a single vehicle suspension system. The best feature of a soft suspension is the smooth ride it provides for the vehicle passengers. The best feature of a stiff suspension is the increased handling performance it provides for the vehicle.
The theory of semi-active suspension systems is to selectively switch between stiff suspension and soft suspension in response to the particular road and driving conditions of the vehicle. Selection between stiff suspension and soft suspension may be obtained by altering the damping force of the suspension system, e.g., a greater damping force for a stiffer suspension and a lower damping force for a softer suspension.
The theory of active suspension system controls is to provide an actuator force to the suspension system to reduce wheel hop and improve vehicle body attitude control beyond that achievable by damping forces alone. The actuator force is applied in equal and opposite directions between the wheel and vehicle body. Active and semi-active suspension systems can be commonly referred to as variable force suspension systems.
Difficulties in designing variable force suspension systems lie partially in system controls. A suspension system may, at any given time, be said to have a state. The suspension system state for a particular quarter of the vehicle includes the position of the vehicle body (the sprung mass), the position of the wheel (the unsprung mass), the velocity of the sprung mass, and the velocity of the unsprung mass. From these four components, the other characteristics of the quarter car suspension system may be determined. For example, the relative velocity between the sprung mass and the unsprung mass is equal to the velocity of the sprung mass subtracted by the velocity of the unsprung mass. The relative position of the sprung mass and unsprung mass is equal to the position of the sprung mass subtracted by the position of the unsprung mass. The relative velocity between the sprung and unsprung masses and/or the relative position of the sprung and unsprung masses may be included in what is referred to below as the relative system state.
In suspension systems, relative displacement between sprung and unsprung bodies may be primarily due to movement of the unsprung mass or primarily due to movement of the sprung mass.
Movement of the unsprung mass is caused by road disturbances and detecting and analyzing these occurrences is difficult. One way is to mount a sensor, e.g., ultrasonic, on the front end of the car to sense the road modulations from a given height. Techniques that require adding ultrasonic sensors to detect road modulations add to the cost of the controllable suspension system on the car. Unless separate sensors are provided, one sensor gives only a local view of the road and does not necessarily represent the road at other corners of the car.
Road conditions cannot accurately be determined by sensing body vertical and/or lateral acceleration signals, since high frequency wheel-hop is filtered by the body.
Another way to detect and analyze movement of the unsprung mass caused by road disturbances is to measure relative motion and use a fast fourier transform algorithm to compute amplitudes of different frequency components of the signals. Use of a frequency analyzer requires a lot of computing power and process delay times., e.g., 2 seconds at least for the 1 Hz. frequency to be detected.
Using the magnitude of the relative velocity between the sprung and unsprung masses to detect frequency can be misleading since these velocity amplitudes can be equally reached through body motion or wheel motion. Measurements at the vehicle center of gravity are only average signals of the four-corner inputs and performance and do not show specific information at a specific wheel.